Many useful applications, such as the detection of radioactive material and computer-assisted tomography (“CAT”) rely on the detection of photon radiation, known as X-ray and/or gamma-ray radiation. Both of these types of high-energy photon radiation cause ionization and for purposes of this disclosure the two terms, X-ray and gamma-ray, are used interchangeably. In terms of the detection of such ionizing radiation, the spectral region of greatest interest for most applications generally falls between the energies of about 20 to 2,000 keV (i.e., 0.02 to 2 MeV).
In the above spectral range of interest, the primary types of interaction are the photoelectric and Compton effects. The relative contribution from each can be determined in quantitative fashion a priori via the combination of the incident photon energy and the atomic number (i.e., Z-number) of the interacting atom. The photoelectric effect describes a single atomic absorption, whereas the Compton effect describes an inelastic scattering collision that simultaneously results in a Compton recoil electron and a Compton scattered photon. The latter can be inelastically scattered again and again, until the photon either exits or is “absorbed” by the interacting media. Of the two processes, the primary basis for the majority of known ionizing radiation detectors used in imaging applications at photon energies up to at least 200 keV is the photoelectric effect, which causes the initial production of a single “free-electron” and a corresponding positive atomic ion.
In order to detect ionizing electromagnetic radiation, several known sensing devices are commonly used. One of the earliest known electronic devices is the ionization chamber. Detection of radiation in an ionization chamber, such as a Geiger-Müeller (“GM”) tube, is based upon electrical conductivity induced in an inert gas (usually containing argon and neon) as a consequence of ion-pair formation.
Further, a number of solid state semiconductors are used for detecting ionizing-photon radiation, the most common of which is the silicon photodiode, which can be either crystalline or amorphous in structure. However, a number of other semiconductor materials have also been used, including Ge, GaAs, CdTe, CdZnTe, etc. The basic principle of operation for semiconductor detectors is similar to the ionization chamber, namely that electromagnetic radiation absorbed by the semiconductor simultaneously creates both electrons and positive holes. The resulting charges move in opposite directions in an applied field, with the current being proportional to the energy of the incident ionizing radiation. The use of large area, amorphous-silicon detectors has recently been employed to produce digital electronic images which can replace X-ray film.
Currently, the most effective known radiation detector is generally considered to be a scintillation counter. Compared to a GM-tube that can have a “dead-time” on the order of 100 μs (microseconds) between counting events, during which time any response to radiation is impossible, a scintillation detector generally has a dead-time of about 1 μs or less. Another advantage of the scintillation detector is that the number of emitted photons produced by the scintillation plate or crystal, upon interaction with ionizing radiation, is approximately proportional to the energy of the incident radiation. However, a conventional scintillation counter requires an expensive crystalline scintillation plate of high optical quality along with expensive photodiodes or photomultiplier tubes.
Based on the foregoing, there is a need for an ionizing-photon radiation detector with high resolution capability, fast pixel response, minimal dead-time, and which can be manufactured in large sizes relatively inexpensively.